This invention relates to magnetic field sensors, and the detection and location of the approximate position of ferromagnetic objects, such as, for example, in the path of a vehicle or of drilling equipment before the vehicle or drill reaches the object, or for measuring cardiac signals.
Interest is increasing in the development of magnetic sensors for sensing low-frequency magnetic fields in terrestrial, extraterrestrial, industrial, biomedical, oceanographic, and environmental applications. The trend in magnetic sensor design and development is constantly toward lower size, lower power consumption, and lower cost for similar or improved performance.
There are several types of magnetometers (magnetic sensors with external instrumentation) currently used. The least expensive and least sensitive devices have resolution of about 1CT1 Oersted (Oe)/Hz½ and typically are Hall effect devices. These devices work by sensing a voltage change across a conductor or semiconductor placed in a magnetic field. Such devices are insensitive and do not lend themselves to applications requiring greater sensitivity, such as that required in brain scan devices and magnetic anomaly detection devices. Magnetoresistive-type magnetic sensors are suited for low-cost, medium-sensitivity applications and have a detectivity of about 0.1-1 nT/Hz1/2 at 1 Hz. Using spin-dependent tunneling magnetoresistive sensors, one can observe more than 100% changes in the resistivity in fields of a few Oe.----.
More sensitive magnetometers exist, but they are typically limited to applications that can tolerate relatively high power size, weight and cost. The most common of these devices are flux gate magnetometers that have a resolution of approximately 0.05-0.1 nT Oe/Hz1/2 at 1 Hz and SQUID (superconducting quantum interference device) magnetometers that have a resolution of about 3-50 fT Oe/Hz1/2 at 1 Hz. Flux gate magnetometers use a magnetic core surrounded by an electromagnetic coil, and are difficult to micro fabricate and have less sensitivity when made very small because the magnetic flux change is smaller. Additionally, flux gate magnetometers require relatively large amount of power and accordingly do no lend themselves to low-cost, compact, portable design. Though SQUID magnetometers are sensitive magnetometers, the apparatus must include a means for cooling to cryogenic temperatures. This makes SQUID magnetometers extremely bulky and expensive to operate. Their size limits their utility because the active superconducting element cannot be placed directly adjacent to the source of the magnetic field, for example the brain. Accordingly, there is need for inexpensive, low power magnetometers that have sufficient sensitivity to be useful for a variety of magnetometer applications at relevant frequencies.
Magnetic sensors used to detect objects that move slowly typically exhibit considerable low-frequency 1/f-type noise (where f is frequency of operation of the magnetic sensor), an unwanted condition. In general, there is a tendency for such devices that have higher sensitivity to also exhibit higher 1/f-type noise. This generally occurs when using magnetoresistive-type magnetic sensors, see van de veer-donk et al. J. Appl. Phys 82, 6152 (1997).
A well-known way of increasing the sensed magnetic field by a magnetic sensor is by use of a flux concentrator, which can enhance a sensed magnetic field by as much as a factor of 50, see N. Smith et al., IEEE Trans. Magn. 33, p. 3358 (1997). An example of such a device is taught in U.S. Pat. No. 5,942,895, entitled “Magnetic field sensor and current and/or energy sensor,” that uses Hall sensors with flux concentrator components. The magnetization of flux concentrators increases in the direction of the field to be measured. This in turn increases the magnetic field flux at the position of the sensor and, thus, increases the output signal from the magnetic sensor.
The magnetization of the flux concentrator can change by domain wall motion or domain rotation. The latter is the preferred mode because it generates less 1/f noise. There are many different ways and materials that can be used for the magnetic material. The overall objective is the largest possible increase in the magnetic field at the position of the sensor.
A magnetic sensor (magnetometer) that addresses 1/f-type noise is taught in U.S. Pat. No. 4,864,237. This disclosure teaches of an apparatus for measuring magnetic fields, which change only at extremely low frequency. This apparatus uses a SQUID magnetometer that includes a superconducting flux transformer that inductively couples a detected signal into a d-c SQUID sensor. This magnetometer can optionally include a device for modulating the detected signal in a frequency range characteristic of low-noise operation of the SQUID. The modulation frequencies are generally above 1 Hz and optionally even above 1-kHz. Limitations of this device include need for cryogenic operation, which inherently do not lend themselves to relatively low cost, low power use.
U.S. Pat. No. 6,501,268, hereby incorporated by reference, by Edelstein entitled “Magnetic Sensor With Modulating Flux Concentrator for 1/f Noise Reduction,” discloses a magnetic sensing device that senses low frequency magnetic fields by using movable flux concentrators that modulate the observed low frequency signal. The concentrator oscillates at a modulation frequency much greater than the observed magnetic field being sensed by the device. The modulation shifts this observed signal to higher frequencies and thus minimizes the effect of the 1/f-type noise, due to the oscillatory motion of a microelectromechanical (MEMS)-type magnetic flux concentrator operated with a magnetic sensor. Modulating of the incoming signal shifts the operating frequency of the sensor. Depending upon the sensor, shifting the operating frequency may reduce the 1/f noise by one to three orders of magnitude at one Hz. At least one modulating flux concentrator is used with the magnetic sensor. Because of its size, this device is especially useful for applications where the space available for the sensor is extremely limited.
U.S. Pat. No. 6,670,809, hereby incorporated by reference, to Edelstein entitled “Magnetic Sensor With Modulating Flux Concentrator Having Minimized Air Resistance for 1/f Noise Reduction,” discloses a magnetic sensing transducer device that senses low frequency magnetic fields by using flux concentrators that modulate the observed low frequency signal, thereby shifting this observed signal to higher frequencies and minimizing 1/f-type noise. This is accomplished by the oscillatory motion of a microelectromechanical (MEMS)-type magnetic flux concentrator operated with a magnetic sensor. At least one MEMS-type fabricated flux concentrator is used with the magnetic sensor. The concentrator oscillates at a modulation frequency much greater than the observed magnetic field being sensed by the device. The device includes a vacuum container for containing the base structure, the magnetic sensor the flux concentrators and the pair of complementary electrodes in a vacuum environment. The device uses flux concentrators that modulate an observed a low frequency magnetic field, thereby shifting this observed field to higher frequencies where the noise of the sensor is smaller to minimize 1/f-type noise. This is accomplished by using a flux concentrator on a microelectromechanical system(MEMS) structure. When the MEMS structure is energized, the oscillatory motion of the flux concentrators modulates the enhancement of the field at the position of the magnetic sensor. The dimensions of the flux concentrators typically encompass gross dimensions of 100 by 75 microns.
U.S. Pat. No. 7,195,945, hereby incorporated by reference, to Edelstein, et al., entitled “Minimizing the Effect of 1/f Noise with a MEMS Flux Concentrator,” discloses a method of method of fabricating a MEMS device that includes forming a magnetic sensor over a SOI wafer which may include an epoxy layer; forming a pair of MEMS flux concentrators sandwiching the magnetic sensor; connecting an electrostatic comb drive to each of the flux concentrators; connecting a spring to the flux concentrators and the comb drive; performing a plurality of DRIE processes on the SOI wafer; and releasing the flux concentrators, the comb drive, and the spring from the SOI wafer. Another embodiment includes forming adhesive bumps and a magnetic sensor on a first wafer; forming a second wafer; forming a pair of MEMS flux concentrators, a pair of electrostatic comb drives, and at least one spring on the second wafer; bonding the second wafer to the adhesive bumps; and compressing the adhesive bumps using non-thermal means such as pressure only.
In the publication Guedes et al. “Low Cost, Low Power, High Sensitivity Magnetometer,” J. Appl. Phys. 103. 07E924 (2008), there is disclosed a technique for removing environmental noise while using a MEMS flux concentrator. The environmental noise is removed by taking the difference between the reading of the sensor and a reference sensor that is separated from the sensor by a distance of 20 feet to one km. According to the publication, magnetic anomaly signals smaller than 1 pT have been detected by this technique upon removing the effect of environmental noise. In the Guedes et al. publication, two flux concentrators were used.
Although attempts have been made minimize the effect of 1/f noise, to completely optimize the magnetic sensors, one must also decrease the magnetic white noise. The white noise is proportional to the inverse of the volume of the ferromagnetic free layer of the sensor. Thus, one would like to use free layers with a large volume. It is difficult to use the MEMS approach to modulate the field in a large volume. Accordingly, there is a need for a magnetic sensor for applications which exceed the capacity of a MEMS structure and which will decrease the magnetic white noise.